Yes, they can swim

Thanks to Mrs Gray's classroom and their report on the groundhog, I can report that groundhogs can indeed swim. Thanks also to my hubby who was intrigued enough with the question to go look for the answer.

The U of M Biodiverstiy Website entry on groundhogs notes "Though observed mostly on the ground, woodchucks have been seen climbing trees to reach food and swimming with only their head and nose above water." This is exactly the behavior I observed.